Unable to reach agreement on the future of plutonium shipments to South Carolina, the state governor and the federal Department of Energy threatened each other with a showdown over the processing of nuclear waste that the government says is vital to national security.

Spencer Abraham, the secretary of energy, sent a letter to the governor, Jim Hodges, offering a written agreement and new legislation ensuring that the plutonium would not be permanently stored in South Carolina after it was processed at the Savannah River Site.

If the governor cannot accept these promises, Abraham wrote, the Energy Department will revoke them and begin shipping the waste into South Carolina.

But Hodges said the government had failed to make a legally binding promise not to store the plutonium in the state, and he demanded that the two sides get a court order enforcing the promise. Without an agreement that the promise was enforceable in court, he will physically prevent the Energy Department’s trucks from crossing the state border.